Proportional only process controller

ABSTRACT

Automatic process controller circuitry for producing an output current which is proportional to a process variable input signal. A differential operational amplifier is connected to receive set point and process variable input signals and provides an output voltage which is proportional to the difference between these two signals. An adjustable proportional band series output impedance is connected between the output of the operational amplifier and a controller circuit output node, and this series impedance converts the output voltage of said amplifier to a current. The voltage developed across this variable series output impedance is differentially fed back via positive and negative feedback loops to the inverting and noninverting inputs of the operational amplifier, and such feedback connection enables the proportional current flowing through the variable series output impedance to be made constant and independent of the output voltage of the operational amplifier. Also, since the load current flowing in the transducer driven by the above controller circuitry can be made equal or proportional to this proportional current, said load current can be adjusted to a value wholly independent of the internal resistance of the driven transducer.

United States Patent Fricke, Jr.

[451 Oct. 3, 1972 [54] PROPORTIONAL ONLY PROCESS CONTROLLER [72]Inventor: LouisH. Fricke, Jr., St. Louis, Mo.

[73] Assignee: Monsanto Company, St. Louis, Mo.

[22] Filed: June 29, 1970 [21] Appl. No.: 50,489

[52] US. Cl. ..330/30 D, 330/69, 330/1 A [51 Int. Cl ..H03f 3/68 [58]Field of Search ..330/30 D, 69; 307/230 [56] References Cited UNITEDSTATES PATENTS 3,467,874 9/1969 Richardson et al. .330/30 D X 3,564,4442/1971 Walsh ..330/30 D 3,434,069 3/1969 Jones ..330/30 D PrimaryExaminer-Nathan Kaufman Attorney-John D. Upham, Harold R. Patton [57]ABSTRACT Automatic process controller circuitry for producingTRANSMITTER IN PUT CURRENT REFERENCE VOLTAGE an output current which isproportional to a process variable input signal. A differentialoperational amplifier is connected to receive set point and processvariable input signals and provides an output voltage which isproportional to the difference between these two signals. An adjustableproportional band series output impedance is connected between theoutput of the operational amplifier and a controller circuit outputnode, and this series impedance converts the output voltage of saidamplifier to a current. The voltage developed across this variableseries output impedance is differentially fed back via positive andnegative feedback loops to the inverting and noninvening inputs of theoperational amplifier, and such feedback connection enables theproportional current flowing through the variable series outputimpedance to be made constant and independent of the output voltage ofthe operational amplifier. Also, since the load current flowing in thetransducer driven by the above controller circuitry can be made equal orpro portional to this proportional current, said load current can beadjusted to a value wholly independent of the internal resistance of thedriven transducer.

4 Claims, 2 Drawing Figures 5+ SUPPLY I /P fRL TRANSDUCER CONTROLLER P20DIFFERENTIAL OPERATIONAL AM PLI FI ER VARIABLE SERIES OUTPUT IM PEDANCEPROPORTIONAL ONLY PROCESS CONTROLLER FIELD OF THE INVENTION Thisinvention relates generally to automatic process controllers and moreparticularly to a proportional only automatic process controller havingimproved adjustable proportional band output current characteristics.

BACKGROUND OF THE INVENTION There are many closed loop servo-typesystems for controlling different variables of a process, and suchsystems normally include apparatus mounted in the process beingcontrolled for measuring a selected process variable (PV). This processvariable is then translated into a corresponding electrical signal andtransmitted to a main process controller where it is compared to a knownadjustable set point reference signal. By such comparison, an errorsignal is generated at the output of the controller, and this errorsignal is in turn utilized to drive a current-to-pressure (I/P)transducer in the closed loop. This I/P transducer actuates a finalcontrol element, such as a pneumatic valve, and this valve controls aselected parameter of the process, such as the process flow rate. Bythis closed loop feedback control, an undesirable deviation in a processparameter may be reduced or eliminated.

In a proportional controller, the electrical characteristics of thecontroller, such as the gain constants, etc., are selected such that aselected percentage of the process variable input signal range willcorrespond to a different selected percentage of the range of controlleroutput currents or voltages. The relationship of these two percentagesis referred to as the proportional band of the controller, abbreviatedP.B. The proportional band RE. is defined as P.B.= LOO/G where G equalsthe gain of the controller. Thus, if the. full range of process variableinput currents (from a transmitter) is from milliamperes to 50milliamperes or a 40 milliampere range, and it is dev sired that adeviation of ,milliamperes within this DESCRIPTION OF THE PRIOR ARTHeretofore, in the construction of automatic process controllers, acommon practice has been to use high gain operational amplifiers in thecontroller circuitry to provide good amplification and sensitivity forthe error signal generated by comparing the process variable and setpoint reference voltages within the controller. With the advent ofintegrated circuits (ICS), the multistage IC operational amplifierbecame particularly attractive for use in process controllers as will beappreciated by those skilled in the art. However, due to the fact thatprocess controllers of the above type must be capable of driving manydifferent types of transducers with widely varying internal resistances,it became necessary to design the operational amplifier and otherassociated controller circuitry so that the controller output current isindependent of the internal resistance of the driven transducer. Thatis, it is desired that the controllers output current be only dependentin some adjustable proportion to the variations in the process variableinput signal, and independent of the internal resistance of the driventransducer.

In order to provide this output or transducer load currentcharacteristic, one prior art process controller circuit utilizes anoperational amplifier and one or more constant current sink outputtransistors connected to the output of the operational amplifier andgrounded through a fixed resistor. By providing a resistive feedbackconnection between the grounded resistor and one input of theoperational amplifier, it was possible to provide a controller outputcurrent which was substantially independent of the internal resistanceof the driven transducer.

The resistive feedback connection mentioned above required that both thefeedback signal and the error signal be applied to one of thedifferential input terminals of the operational amplifier and that itsother differential input terminal be connected to ground. The latterrequirement necessitated the use of a separate differential amplifierstage connected intermediate the above operational amplifier and the setpoint and process variable input signals for the controller. Thisadditional differential amplifier stage was used to generate the errorsignal which was then applied along with feedback signal to one inputterminal of the output operational amplifier.

Alternatively, and in place of the second differential amplifier stagementioned above, a separate D.C. power supply could be connected betweenthe process variable input terminal for the controller and the outputoperational amplifier stage for generating the required set point signalto be summed with the process variable input electrical signal. Ineither case above, however, one of these two additional electroniccomponents was required at the input of the output operational amplifierstage where the latter was connected with resistive feedback aspreviously described.

SUMMARY OF THE INVENTION The general purpose of this invention is toprovide a proportional band controller which possesses all of theadvantages of similarly employed prior art process controllers, but doesnot require either the combination of separate, cascaded amplifierstages or a single amplifier stage and a power supply in seriestherewith to maintain a constant controller output current for driving atransducer. To attain this, the present invention utilizes a uniqueoutput impedance and feedback connection for a single differentialoperational amplifier. This novel connection according to the presentinvention permits the single differential operational amplifier of thecontroller to differentially receive both set point and process variableelectrical signals, thereby eliminating the need for either a separatedifferential amplifier stage for generating an error signal or aseparate power supply for generatinga set point signal.

Accordingly, an object of the present invention is to provide a new andimproved automatic process controller having an adjustable proportionalband.

Another object of this invention is to provide a new and improvedautomatic proportional only controller utilizing a single differentialoperational amplifier which produces a constant, adjustable outputproportional band current. This current is independent of either theinternal resistance of the driven transducer or the output voltage ofthe operational amplifier.

A further object of this invention is to provide a new and improvedautomatic proportional band process controller which is simple and easyto construct, relatively low in cost and reliable in operation.

Briefly, the automatic controller according to the present inventionincludes an operational differential amplifier having a variable seriesoutput impedance for converting the output voltage of the operationalamplifier to a current which is proportional to a process variable inputsignal. The voltage developed across this variable series outputimpedance is differentially fed back to the inputs of the operationalamplifier via positive and negative feedback loops, so that theproportional output current of the controller is independent of theinternal impedance of the transducer driven by the controller andindependent of the absolute value of the output voltage of theoperational amplifier. Anadjustable current source is connected to acontroller output node and may be variably biased in accordance with thedesired DC. bias current for the controllers output load (transducer)current range.

DRAWINGS FIG. 1 is a functional block diagram of the process controlclosed-loop system utilizing the present invention, and

FIG. 2 is a schematic diagram representation of a circuit embodiment ofthe proportional band controller according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, there isshownfunctionally a process in which a process variable parameter, such as aflow rate, is being controlled. A primary element 12 responds to theprocess variable being controlled and provides an electrical inputsignal to a transmitter 14. The transmitter 14 develops a desired rangeof process variable input currents or voltages which are applied viaconductor 16 to the automatic process controller 18. The automaticprocess controller 18 compares the process variable input signalonconductor 16 with a fixed set point reference potential applied toterminal 31, and as a result of such comparison, generates an outputerror signal on conductor 20. This output error signal may either be acurrent or a voltage, depending upon the requirements of the transducerstage 22 driven by thatautomatic controller 18. In the embodiment of theinvention to be subsequently described, the automatic controller 18output signal via line 20 is a load current which is convertedto apressure in the UP transducer stage 22. This pressure is in turn used tocontrol the final control element 24, such as a control valve (notshown). The final control element 24 returns the measured parameter ofthe process 10 to a desired value or range as is well-known in theprocess control art.

The automatic controller 18 in FIG. 1 isillustrated in schematic detailin FIG. 2 and is embodied as a proportional band controller whose outputcurrent I, is proportional to the process variable input current flowingin line 16. That is, a full range of output current I flowing fromtransducer 22 into the controller output node is proportional to aselected percentage of the full range of transmitter input currentflowing through input conductor 16. The transducer 22 and controller 18can be powered by the single B+ supply connected at supply terminal 23or separate supplies (not shown).

The transmitter input current flows through an input resistor 26 anddevelops thereacross a process variable input voltage. This processvariable input voltage is compared in the controller 18 to a set pointvoltage derived at variable tap 31 on the set point resistor 30. Theprocess variable and set point voltages which are available at terminals32 and 34, respectively, may be switched through the reversible switchconnections 36 and 38 to the respective terminals 40 and 42 in thecontroller circuitry. Upon moving switches 44 and 46 to their closedposition as shown in FIG. 2, the set point voltage at terminal 42 isconverted to a set point reference current by a first input resistor 48and is applied to a first input terminal 66 of the differentialoperational amplifier 52. Upon closure of switch 46, a second inputresistor 50 converts the process variable input voltage at terminal 40to a process variable current which is applied to a second inputterminal 68 of the operational amplifier 52. The letter designations Rand R for the first and second input resistors 48 and 50, respectively,the other resistor letter designations R R R and the signal voltagedesignations e e e, e, c and e,,, will be discussed in a subsequentportion of the specification This subsequent portion of thespecification is directed to the operation of the controller circuit 18with the current source switch 73 open and the current I equal to zero.The description of such connection and associated circuit operation willclearly establish the relationship between transducer 22 load currentand the differential input voltages e' and e" at terminals 66 and 68respectively.

These first and second input terminals 66 and 68 are referred to as theinverting and noninverting inputs of the amplifier 52, and thisamplifier 52 may advantageously take the form of an integrated circuitoperational amplifier. The operational amplifier 52 may be readilyselected from many commercially available types of operationalamplifiers, and a typical integrated circuit operational amplifier 52,the following detailed portions of which are not shown in the drawing,often includes a first differential amplifier stage that provides mostof the amplifier circuit gain. An intermediate differential stage isusually included to provide some additional gain where such is required,and a DC level shifting or level translating stage is commonly connectedto the output of the intermediate stage to remove undesired DCcomponents which are introduced into the amplified signal in twopreceding stages. An output current amplifier stage is typicallyconnected to this translating stage to provide the required outputvoltage swings and the required output current drive capability for theoperational amplifier. For improved frequency compensation, externalcapacitors are frequently connected between stages or to individualtransistors in a single stage of the operational amplifier 52.

The operational amplifier 52 provides an output voltage e at its outputterminal 69 which varies in proportion to the very small voltagedifference between the input voltages e and e" at the terminals orconnections 68 and 66, respectively. This output voltage e at terminal69 is fed back via a first feedback resistor 54 to the first amplifierinput terminal 66. A variable series output impedance 56, which includesa fixed resistor 58 and a variable shunted resistor 60, converts thisoutput voltage e at terminal 69 to an output current, I which flows froma circuit output node 70. A second feedback resistor 64 is connectedbetween the output node 70 and the second amplifier input terminal 68,and the ohmic values of the first and second feedback resistors 54 and64 and the first and second input resistors 48 and 50, respectively, setthe gain of the amplifier 52 as is well known and understood by thoseskilled in the art. In the embodiment of the present inventionillustrated in FIG. 2 which has been built and successfully operated,feedback resistors 54 and 64 are equal in ohmic value and inputresistors 48 and 50 are equal in ohmic value.

The set point current which flows through the first input resistor 48develops a very small voltage 2' with respect to ground in the fractionof a millivolt range at a first input terminal 66 of the differentialoperational amplifier 52. Similarly, the process variable currentflowing through the second input resistor 50 develops a very small inputvoltage 2 at a second input terminal 68 of the operational amplifier 52,and the difference between these very small input voltages e and e isamplified in the operational amplifier 52. Since the input currentsflowing into the operational amplifier 52 from input terminals 66 and 68are negligible, the direction of the current through the first feedbackresistor 54 is from terminal 66, through feedback resistor 54 and intothe amplifier output node 69. Similarly, the process variable currentwhich flows through the second input resistor 50 also flows fromterminal 68, through the second feedback resistor 64, into thecontroller circuit output node 70, through the variable output impedance56 and-to the amplifier output node 69. Neglecting the very small valueof load current I, for large transducer loads, the current I flowing inthe variable series output impedance 56 is approximately proportional tothe difference between the positive and negative feedback voltagesapplied viathe first and second feedback resistors 54 and 64 in therespective positive and negative feedback loops to the input terminals66 and 68 of the operational amplifier 52. Therefore, it is seen thatthe proportional current I is substantially independent of the absolutevalue of the output voltage e, at the operational amplifier output node69. By simply adjusting the variable tap 62 on the potentiometer 60, thevalue of the variable series output impedance 56 may be varied so that Iwill vary in any desired proportion to variations in the transmitterinput current flowing in conductor 16. The small fixed resistor 58 isincluded in the variable output impedance 56 as a safety precaution toprevent the potentiometer tap 62 from removing all series outputresistance at the output node 69 of the operational amplifier 52.

For a further understanding of the proportional band operation of theprocess controller 18 according to the present invention, considertypical input current values for the transmitter input current rangingfrom 4 milliamperes to 20 milliamperes, or a range of 16 milliamperes.Therefore, 16 milliamperes represents 100 cent of the input currentrange for the controller. If it is desired that this 16 milliamperes ora full range of input currents correspond to a l-5 volt range of outputvoltages developed across variable series output impedance 56, then inaccordance with Ohms law, the variable output resistor 60 should beadjusted (neglecting the small fixed resistor 58 which is typically onthe order of 10 ohms) to approximately 250 ohms. The

above current and voltage values correspond to a proportional band of100 percent and a gain of 1.

On the other hand, if it is desired to operate the controller 18 with aproportional band PB of 25 then 100 milliamperes of current I wouldcorrespond to 1 volt drop across the variable output impedance 56. Inthe latter example, the variable output impedance would have to beadjusted to l/lOO X 10 or approximately l0 ohms.

Frequently, it is desired that the output transducer current I of thecontroller 18 be biased around an operating point; that is, the value ofthe output current I flowing into the circuit output node be adjustedequal to some constant, K +1 This is accomplished in accordance with thepresent invention by the use of an adjustable current source 71,including a transistor 72 which is connected through a base biasresistor 76 to a reference voltage terminal 74. A current limitingresistor interconnects the emitter of the NPN current source transistor72 to ground potential. By varying the position of the movable base tap78 on resistor 76, the operating bias for transistor 72 may be changedto either increase of decrease the value of transistor collector currentI which is the constant K in the above question. In this manner, themidpoint or bias point within the current range for I, may beconveniently varied independently of the proportional band variation ofI, as previously described.

It has been stated above and will be mathematically proven below thatthe current I flowing in the series output impedance 62 (or R isproportional to the difference between the set point and processvariable input signals e, and e Additionally, it will be shown that thecurrent I is approximately equal to the transducer load current I whenthe current source current I is equal to zero with switch 73 open.

If e, is defined as the set point signal voltage between input terminal42 and ground, andve is defined as the process variable signal voltagebetween input terminal 40 and ground, then, by superposition, e may beand e may be written as Where e, is the voltage at the circuit outputnode 70. If the gain of the operational amplifier 52 is defined as aconstant A, then the output voltage e at node 69 may be written asCombining equations 1 and 2 above, the value e'-e" for the amplifieddifferential voltage may be expressed Substituting e lA for the quantity(e"e') (Equation 3 above), the output voltage e, at node 69 may bewritten as R3 R4 [R.+R. R2+R4 l 1 [RI+R3* RZ+RK By factoring Eq. 5above, the following expression results If both sides of Eq. 6 above aredivided by the gain A of the operational amplifier 52, then equation 6above may be modified as 2+ 4' 1 r-la q- .2v If the gain A of theoperational amplifier 52 is very high, in the order of 4 X for example,then 11A is approximately 0. If HA is assumed to be equal to 0 forpresent purposes, then Eq. 7 above may be modified as e 1 1 i 1 121+ R.L R2+Rl 2 R2+R4 R1+R3 q- If the resistors R R R and R are equal, thenequation 8 may be written as [2 (e,,e /2 (e e or a r.)=( z r) (Eq. 9Therefore, from Eq. 9 above, it will be seen that the voltage (e -1developed across the series output impedance 56 is equal to thedifference between the set point and process variable input signals e,and e If, for example, R R 10R, IOR then Eq. 9 may be modified above as1/11 (e -er) 10/11 (82 e )or (e eL) l0(e2e H 7 (Eq. 10) Thus, inaccordance with the examplary E51. 10 above, the voltage differenceacross the series output impedance 56 can be adjusted to a value 10times the difference between input signals e and e, and this changes theproportional band (PB) range by a factor of 10.

From Eq. l0 above, it will be understood that a general expression forinput and output voltage relationships for the controller 18 can bewritten as r r r- 1) where K is the. proportional band range of thecontroller and is equal to R /R and R /R The proportional currentl can now be defined as I L 2- 1) 2 R,, R, (Eq. 12),

where R, is the series resistance of the series output im pedance 56.

If the feedback resistor R is much greater than R, (i.e. R R,), then thefeedback current I, through resistor R is a negligible fraction of theload current 1 For example, R is typically 200 Kilohms whereas R, rangestypically from 100 ohms to 250 ohms. Therefore, 1 may be in the order ofl/ l ,000 I Thus, neglecting 1 and assuming that l l with the switch 73'open, then the relationship between the transducer load current 1 andthe set point and process variable pqtv lta ssn ndez. ma bqsx r s as I Nm 1:. @943) which is the total gain equation for the controller 18 withthe switch 73 open as previously described. And, as described in detailhereinabove, R, is made adjustable to provide fine proportional controlof the load current 1,, and K can be varied by changing the input andfeedback resistors R R R and Rgto change the proportional band range ofthe controller 18.

Various modifications may be made to the embodiment of the presentinvention illustrated in FIG. 2 without departing from the true scope ofthe invention. For example, various types of current sources whichprovide a constant and adjustable current 1;, may be substituted for.the current source 71 without departing from the true scope of thisinvention. Similarly, the ratio of the feedback and input resistors foramplifier 52 may be changed as previously mentioned to vary the gain ofthe differential operational amplifier 52 within the scope of thepresent invention.

The following table includes ohmic values for the resistor componentsused in a circuit of the type described which has been built andsuccessfully operated in the closed loop process environment for whichit is intended. However, said table should not be construed as limitingthe scope of this invention.

TABLE COMPONENTS VALUE OR TYPE Resistor 26 250 ohms 3i i l kilohm 48 200kilohms 50 200 kilohms 54 200 kilohms S8 10 ohms 60 2500 ohms 64 200kilohms 76 l kilohm 80 250 ohms B+ Supply Voltage 24 to 30 volts Iclaim:

d. a first conductive feedback path connected to one point on saidoutput resistance and between said output terminal of said differentialamplifier means and said first input terminal thereof, and

e. a second conductive feedback path connected between another point onsaid series output resistance and said second input terminal of saiddifferential amplifier means, so that the voltage differentially fedback to said first and second input terminals of said differentialamplifier means is that voltage developed between said first and secondpoints on said series output resistance, whereby the proportional outputcurrent flowing through said series output resistance and through saidoutput circuit node is independent of the internal impedance of thetransducer connected to said output circuit node and driven by saidcontroller, and is further independent of the absolute value of theoutput voltage at said output terminal of said differential amplifiermeans.

2. The controller defined in claim 1 wherein:

a. said first conductive path includes a first feed-back resistorconnected between said output terminal of said differential amplifiermeans and said first input terminal thereof,

b. said second conductive feedback path includes a second feedbackresistor connected between said circuit output node and said secondinput terminal of said differential amplifier means, said input circuitmeans further includes,

c. a first input resistor connected between a set point input terminaland said first input terminal of said differential amplifier means, and

d. a second input resistor connected between a process variable inputterminal and said second input terminal of said differential amplifiermeans, whereby said first and second input terminals of saiddifferential amplifier means serve as first and second summingjunctions, respectively, for said set point and process variable inputsignals, and the ratio of said first input resistor to said firstfeedback resistor and the ratio of said second input resistor to saidsecond feedback resistor establishes the proportional band current rangeof said controller.

3. The controller defined in claim 2 wherein said series outputimpedance includes a fixed resistor portion and a variable resistorportion, so that current flowing through said series output impedancemay be varied in accordance with the desired proportional band outputcurrent flowing into said circuit output node.

4. The controller defined in claim 1 which further includes anadjustable constant current source connected between said circuit outputnode and a point of reference potential, said constant current sourceconducting a current approximately equal to the difference between thecurrent flowing into said circuit output node and the current flowingthrough said series output resistance to thereby permit adjustment ofthe output DC bias current flowing into said output circuit node fromthe stage driven thereat by said controller.

1. A process controller for producing an output current proportional tochanges in a process variable input signal applied thereto, including incombination: a. input circuit means for receiving process variable andset point input signals, b. differential amplifier means having firstand second input terminals thereof connected to said input circuit meansfor differentially receiving and amplifying said process variable andset point input signals, c. a series output resistance connected betweenan output terminal of said differential amplifier means and an outputcircuit node, d. a first conductive feedback path connected to one pointon said output resistance and between said output terminal of saiDdifferential amplifier means and said first input terminal thereof, ande. a second conductive feedback path connected between another point onsaid series output resistance and said second input terminal of saiddifferential amplifier means, so that the voltage differentially fedback to said first and second input terminals of said differentialamplifier means is that voltage developed between said first and secondpoints on said series output resistance, whereby the proportional outputcurrent flowing through said series output resistance and through saidoutput circuit node is independent of the internal impedance of thetransducer connected to said output circuit node and driven by saidcontroller, and is further independent of the absolute value of theoutput voltage at said output terminal of said differential amplifiermeans.
 2. The controller defined in claim 1 wherein: a. said firstconductive path includes a first feed-back resistor connected betweensaid output terminal of said differential amplifier means and said firstinput terminal thereof, b. said second conductive feedback path includesa second feedback resistor connected between said circuit output nodeand said second input terminal of said differential amplifier means,said input circuit means further includes, c. a first input resistorconnected between a set point input terminal and said first inputterminal of said differential amplifier means, and d. a second inputresistor connected between a process variable input terminal and saidsecond input terminal of said differential amplifier means, whereby saidfirst and second input terminals of said differential amplifier meansserve as first and second summing junctions, respectively, for said setpoint and process variable input signals, and the ratio of said firstinput resistor to said first feedback resistor and the ratio of saidsecond input resistor to said second feedback resistor establishes theproportional band current range of said controller.
 3. The controllerdefined in claim 2 wherein said series output impedance includes a fixedresistor portion and a variable resistor portion, so that currentflowing through said series output impedance may be varied in accordancewith the desired proportional band output current flowing into saidcircuit output node.
 4. The controller defined in claim 1 which furtherincludes an adjustable constant current source connected between saidcircuit output node and a point of reference potential, said constantcurrent source conducting a current approximately equal to thedifference between the current flowing into said circuit output node andthe current flowing through said series output resistance to therebypermit adjustment of the output D.C. bias current flowing into saidoutput circuit node from the stage driven thereat by said controller.